WAEC - Physics (2000 - No. 36)
An immersion heater is rated 120W. How long does it take the heater to raise the temperature of 1.2kg of water by 15oC. (Assuming heat lost to the surrounding is negligible. Specific heat capacity of water = 4200 Jkg-1K-1C)
65.0 minutes
60.0 minutes
52.5 minutes
10.5 minutes
Explanation
Given:
P = 120W, t = ?, m of water = 1.2kg, c of water = 4200Jkg\(^{-1}\)K\(^{-1}\), ΔT = 15ºC
Q = pt = m c ΔT
120 x t = 1.2 x 4200 x 15 = 75600J
t = \(\frac{75600}{120}\) = 630secs → 10.50minutes.